The antenna of an active phased array system must be able to steer its beam so that the system can obtain information about the surroundings in different directions. It is also desirable that the antenna suppress signals from other directions than the direction in which the system is currently transmitting and receiving. A phased array antenna comprises a number transmitting/receiving elements, usually arranged in a planar configuration. Each element, or a group of elements, is driven by a transmit/-receive (T/R) module which controls the phase and the amplitude of the corresponding antenna element.
On transmission of a signal from a phased array antenna, the signal is divided into a number of sub-signals, and each sub-signal is fed to one of the modules. The modules comprise signal channels guiding the sub-signals to the antenna elements. Each signal channel comprises controllable attenuators or amplifiers and controllable phase-shifting devices for controlling the amplification and the phase shift of the modules. The signals transmitted through the antenna elements interfere with each other. By selecting suitable values of the relative amplification and the relative phase-shifting between the modules and by utilizing the interference of the transmitted signals, the directional sensitivity of the antenna can be controlled.
During reception in a phased array antenna, the opposite procedure takes place compared to transmission. Each antenna element receives a sub-signal. The modules comprise signal channels for reception and through these signal channels the sub-signals are collected in a single point in which all sub-signals are added to form a single composite signal. The signal channels for reception also comprise amplifiers and phase shifters, and the directional sensitivity of the antenna for reception can be controlled in a corresponding way as for transmission, by varying the amplification and phase-shifting of the modules.
In order to obtain the desired directional properties of the antenna, it is necessary to minimize the side lobe levels of the antenna. To enable low side lobe levels with an electrically controlled phased array antenna, high accuracy of the amplification and the phase shift in the modules is required. In practice, this is achieved by introducing a calibration function in the antenna system. Central to the calibration concept is the compensation of the various contributions of cables, attenuators, phase shifters, regulators and other parts in the transmit/receive channels which respond differently at different temperatures, for each antenna element and at each radio frequency. The calibration procedure is required to determine what controls should be applied to the transmit/receive modules in order to obtain the desired current distribution on the antenna aperture.
For example, if it is required that the phase of the signal fed to all antenna elements be identical, but it is found during calibration that, owing to mismatches in the phase shifters coupled to first and second antenna element, there is a phase difference between the signals output by a first antenna element and a second antenna element of +15°, then the phase shift signal that is fed to the second antenna element must have a phase offset of −15° relative to the phase shift signal fed to the first antenna in order to compensate for the mismatch in the two phase shifters. Differences between the amplitudes of signals that are output by different antenna elements caused by mismatches in the gains of the amplifiers coupled to the antenna elements are compensated for in a similar manner by applying different gain offsets to the antenna elements relative to a given reference antenna element.
Phased array antenna architectures typically include a calibration network, whose purpose is to provide injection of a predetermined calibration signal to each antenna element and to the T/R module connected to it. Such a calibration network is shown in U.S. Pat. No. 7,068,218 (Göttl et al.) which describes a calibration device for an antenna array, or an improved antenna array, that can be viewed as a set of RF-couplers (one coupler per antenna element) interconnected and driven by a passive network having a common feed point. The passive network splits the drive signal in a predetermined manner so that the signal fed to each antenna element is known in advance and the phase and gain offsets are known and predetermined.
During use, one or more antenna elements may become out of calibration. This can occur, for example, owing to one or more antenna elements being replaced. Since the replacement antenna elements will inevitably have slightly different properties to the original antenna elements, the original offsets will not compensate for slight differences in the phase and gain characteristics of the phase shifters and amplifiers used to feed steering signals to the replacement antenna elements. This typically requires that the complete phase antenna array be returned to the factory for re-calibration in order to establish the new offsets. It is also known to perform the re-calibration procedure in the field, but this then requires a calibration network for which the required offsets are known for each phase shifter and amplifier. Such calibration networks are available but they require sophisticated electronics and are expensive.
U.S. Pat. No. 7,068,218 Göttl et al. discloses a calibration procedure that utilizes, in addition to the operational transmit/receive channels, also an auxiliary injection network, whose contribution must be known in advance. This is determined using the concept of the calibration ratio, which measures the ratio between signals injected externally (in principle from infinity) to those injected internally.
Some antennas are factory calibrated. When deployed, the quality of the calibration is tested by one means or another and if the test fails the antenna is sent back to the factory for recalibration. Other antennas have field calibration capability. A number of approaches for calibration of such antennas have been proposed in prior art.
There is a vast literature of prior art relating to phased antenna calibration and the determination of calibration ratio. Of the many different approaches that are known in the art, all presently fall into one of two categories. Some methods use an external calibration signal that is disposed at infinity so that the respective amplitudes and phases of the external calibration signals injected into each antenna element are the same. This, of course, greatly simplifies the determination of calibration ratio, but is not feasible when there is insufficient space between the external calibration source and the phased array antenna, such as when a phased array antenna is recalibrated in the field.
The other approach disposes the external calibration source proximate each antenna element in turn, while ensuring that the distance from the external calibration source to each antenna element is the same and that the external calibration source is exactly aligned to the optical center of each antenna element. This also ensures that the respective amplitudes and phases of the external calibration signals injected into each antenna element are the same, but requires critical and consequently complex alignment and is both time-consuming and expensive.
Replacement of a failed T/R module during antenna maintenance is a routine procedure, which requires recalibration of the antenna system. The amplification and phase shift of the T/R modules are obtained by considering the change in amplitude and phase of the test signal when it passes the T/R module. The control signals controlling the attenuators and the phase shifters in the T/R modules can now be corrected so that the amplification and the phase-shift are made to coincide with the desired amplification and phase-shift.
In accordance with the calibration procedure of plane array antennas in a production environment as taught by above-mentioned U.S. Pat. No. 7,068,218 (Göttl et al.), for example, a plane wave RF-source is used to simulate a point RF-source at infinity. If the propagation direction of the plane wave is parallel to the bore sight axis of the plane array, all array antenna elements are in the same phase conditions. This means that ideally measured phase values of the signal received by all array antenna elements are identical since each pair of array antenna elements and T/R module is assumed to be identical. The calibration procedure enables amplitude and phase characteristics of each pair of antenna element and T/R module to be determined.
When the calibration reference signal is derived from a distant source such as a satellite, the signal emanates from infinity so that its wavefront is effectively equidistant from all the antenna elements. It therefore arrives in the same phase at all the antenna elements. But it is not always practical to use a distant source for the calibration source, particularly when space is at a premium as is often the case in field calibration. Prior art approaches that employ so-called near field calibration are known to feed a planar calibration signal successively to the antenna elements. For example, U.S. Pat. No. 6,084,545 (Lier et al.) discloses a near-field calibration arrangement for a phased-array antenna that determines the phase shifts or attenuation of the elemental control elements of the array. The calibration system includes a probe located in the near field, and a calibration tone generator. According to the concept of reciprocity, the near field calibration procedure can be applied to transmit or receive modes as well. In case of receive calibration mode, a probe sequentially moves from one antenna element to another, keeping the same coupling conditions (distance from antenna plane, polarization, orientation etc.) and transmitting the same test signal. A receive antenna array has a switching arrangement, providing appropriate RF-module/antenna element connection to the measurement unit via controllable phase shifter/attenuator. The near-field calibration goal achieves the same signal parameters (phase and amplitude) coming from each RF-module (and appropriate probe locations) by applying control signals to the appropriate phase shifters and attenuators.
It should also be noted that regardless of whether near field or far filed calibration is performed, when a calibration network is factory-calibrated, sets of calibration values must be pre-assigned to each antenna. These values cannot be determined in the field and are apt to be inapplicable to a replacement antenna element, so that if an antenna element is replaced in the field, such an approach is fraught with difficulty.
In summary, far field calibration allows the calibration signal to be fed simultaneously to all the antenna elements from a common source and ensures that it will arrive at the same phase at all the antenna elements; but is not suitable for use in confined spaces, such as when re-calibrating antenna elements in the field. On the other hand, near field calibration requires that in order for the external calibration signal to arrive at the same phase at all the antenna elements, it must be fed to each antenna element sequentially and this requires precise alignment which is time-consuming and expensive.
It would therefore be desirable to combine the advantages of both approaches so as to calibrate the antenna elements using an external calibration signal that is fed from a common source that is proximate the antenna elements so as to reach all the antenna elements simultaneously, while nevertheless correcting for the fact that the external calibration signal arrives at different phases to each of the antenna elements.